In the field of semiconductor packaging, the use of multilayer ceramic (MLC) technology is widespread. In a typical package, the semiconductor chips are mounted on one surface of the multilayer ceramic substrate. The other surface of the substrate is provided with I/O pins or connectors for mounting or interconnection with the next level of packaging. Throughout the substrate a network of metallurgy is fabricated for interconnection from the pins to the integrated circuit devices that are mounted on the top surface. The I/O pins are ordinarily bonded, by a braze or solder process, to a bonding pad which is also the termination pad of the internal metallurgy system. The termination pads are formed in a conventional method, such as by screening the metal pastes through a mask prior to the sintering thereof, or by metallization such as, by electron beam evaporation, sputtering or other methods after sintering. The I/O pins are then bonded to the termination pads, usually by means of a Au-Sn braze. The bonding material must be sufficiently strong to withstand the environmental conditions met in operation; and, the braze alloy should also be such that is not significantly affected by the thermal and tensile stresses associated with the processing steps undertaken for completion of the package, such as the device mounting procedures. Since the multilayer ceramic substrates are designed to carry a large number of devices, there is ordinarily a need for several re-work steps to allow for device defects and engineering changes. The bonds between the I/O pins and the termination pads must therefore withstand many temperature cycles in the course of the processing and re-working stages. Beyond the fabrication considerations, tensile stresses are created in the vicinity of the I/O pads during usage and have been found to be dependent upon the size, thickness and geometry of the termination pads, the material properties of the pads, the pin joint morphology, and the distribution and material properties of the braze alloy. With this knowledge, various approaches to stress relief in the I/O pin region have been explored. With regard to the thermal stresses, a materials-related approach has been attempted in order to match the thermal expansion coefficients of the associated materials. For example, Kovar (a trademark of Westinghouse Electric Corporation) is commonly used as a pin material as it has a thermal expansion coefficient which is compatible with that of an alumina substrate. The braze and pad materials can be altered in order to provide intermediary TCE's between the substrate and the pin materials in order to reduce the thermal stresses, as taught in U.S. Pat. No. 4,418,857 to Ainslie, et al and U.S. Pat. No. 4,518,112 to Miller et al. In addition to the braze alloy material modification, the Ainslie and Miller patents, both assigned to the present assignee, teach that the braze or fillet volume and the fillet morphology can affect the strength of the braze joint. The Miller patent seeks to reduce the volume of the fillet material to avoid the "tendency to creep upwardly reducing the strength of the joint . . . " (Column 1, lines 63-68). The Ainslie patent teaches a process which " . . . prevents the pin climb of metal up the shank of a pin . . . " (Column 5, lines 35-36). Similarly, the fillet amount and fillet morphology is discussed in IEEE article by Sahara, et al entitled "Improvement of Metallization for Alumina Substrates", Electronic Components Conference Proceedings, May 1982, pages 32-35, wherein it is taught that the recommended volume of bonding material is the minimum volume necessary to achieve the bond. As further illustrated in that article, at page 34 in FIG. 8, the stress profile for the reduced fillet volume sample (at [1]) is far more favorable than the stress profile for the higher fillet volume sample (at [2]). Still another reference, U.S. patent application Ser. No. 626,185, entitled "Process for Bonding Current Carrying Elements To a Substrate in an Electronic System, and Structures Thereof", which has now issued as U.S. Pat. No. 4,634,041, teaches the use of a pin head having a diameter which is significantly smaller than the diameter of the bonding pad in order to decrease the volume of the braze alloy and therefore decrease the likelihood of migration of the braze alloy. All indications from these sources are that the ideal pin joint has a low volume of the braze deposited between the pin head and the bonding pad and not extending in any direction beyond the periphery of the pin head itself. During the re-work procedures, however, the braze materials will migrate, the braze volume will be redistributed and the braze/fillet morphology will shift thereby increasing the stresses.
It is therefore an objective of the present invention to provide a method for bonding connectors to substrates whereby the stresses created in the ceramic are greatly reduced.
It is a further objective of the invention to provide a connector which provides minimum stress to the bonding pad and the underlying ceramic.
It is another objective of the invention to minimize the braze thickness gradient thereby more evenly distributing the stresses across the entire ceramic contact surface.
It is still another objective of the invention to provide a connector geometry which will prevent the redistribution and migration of the bonding material during further processing steps.
It is a further objective of the invention to provide a connection system which will increase the peeling and bending resistance of the pin joint.
These and other objectives will be met by the present invention wherein a connector is taught which has a shank portion, a flat head for connection, and a tapered portion extending from the flat head to the shank.